There’s plenty of helpful replies in this thread already, so I won’t go too much in-depth. But I will tell you that I’ve played this game on and off since 2005, and the very DNA of the game has been structured in a way that breeds hostility and competition between players. Things like ore/herb nodes being first-come, first-serve encouraged griefing behaviours, for an example. Small things. Tiny things. But very many of them. Only very recently (I think from late WoD/ early Legion) has game design begun favouring social harmony between players. It’s a very slow process, and there are large parts of the playerbase that actively resist such change - and there’s still a long way to go.
I think Blizzard were forced into adapting in part due to their competitors like Guild Wars 2 and Final Fantasy XIV being more pro-social in their design, but also due to workplace issues that have come to light recently.
As a veteran player, I’ve gone from racing for world firsts with Ahn’qiraj and Naxxramas to being a very, very (very) casual player, and I don’t touch m+ content, because that’s a timed version of a normal dungeon, where you are racing against the clock, and other players may be very unforgiving towards mistakes. You also have to remember that behind other players is a human being, and not everyone is a mature adult with a daytime job. There are angry children and angsty teenagers playing the game. There are kind and gentle seniors playing the game. There are students, artists, lawyers, housewives and widowers playing the game.
You have to consider what kind of player plays what kind of content, and you’ll never be able to do more than guess, because we’re all hiding behind avatars. But do consider what kind of people will have enough time on their hands to invest in doing mythic+ content, and what they gain from doing so (and what it means to them to be able to finish it), and you might also get an idea of why failure is not an option, and why they might react strongly towards players they feel are threatening their accomplishments and ability to finish.
If you want to keep doing hardcore content, all the more power to you, but I’d say play with your boyfriend, make friends in the game, join a social guild, and rely less on random people. Just like in real life.
Good luck!